Electric hygroscope



United States Patent Ofiice 2314,34? Patented July 26, 1955 ELECTRICHYGROSCOPE Leo S. raig, Fair Haven, and Alien Kaufman, Elberon, N. .l.,assignors to Monmouth Electric Company, Neptune, N. J.

No Drawing. Application June 2, 1953, Serial No. 359,214

6 Claims. (Cl. 201-63) This invention is related to our co-pendingapplication Serial No. 353,877, filed May 8, 1953.

This invention relates to hygroscopes such as an element whoseelectrical resistance changes with humidity and in which the resistanceof the element may be calibrated to determine the approximate humidityof the air surrounding the element and more particularly to a carbontype resistance hygrometer.

In the past hygroscopes have been developed to detect humidity and theyhave taken various forms, for instance, by applying a human hair to aninstrument, a pointer was moved over a calibrated scale to indicateapproximate humidity. Another type of visual instrument is to indicatethe proximity of dew point in atmosphere or gases. A further developmentwas to indicate change in barometric pressure within an air tightcontainer as a measure of the moisture content. The latest inhygroscopes has been with a carbon type humidity element in which theeiectrical resistance of moisture bearing material (such as carbon)varies directly as the moisture content thereof. The carbon type coatingdeveloped and utilized consists of a dilute water solution of: carbon,rydroxyethyl cellulose, polyoxyethylene, sorbitol and alkyl arylpolyether alcohol sprayed on a fiat plastic, glass or ceramic surfacewith metallic conducting edges or electrodes until the proper resistanceis obtained. This sprayed film is in effect a hygroscope in that itsresistance changes directly to the moisture absorption of the element.This carbon film or paste may be similarly dispersed on a strip of glassor plastic by dipping or painting and metal electrodes provided ateither end thereof. The material in the film as described above includesthe carbon powder as the conductive material. The remaining ingredientsare the dispersing agents and binders and humectant. The action ofhumidity upon this film or paste is in effect to move the carbonparticles further apart or closer together thus changing the resistanceof the film. The limitations or deficiencies of this type of carbonelement are (1), in order to obtain a sufiiciently low resistance forthis particular use, a large amount of carbon must be used. With thistype of unit it is essential that the unit be fairly or reasonablysmall. Thus, with a small unit the film thickness becomes so great thatin inducing an electrical current through the thick film there is rathera large lag in the reading. To overcome this lag and to obtain a lowlag, the unit must be made with a very large surface area, thus, itsdimensions are beyond the size permissible for this type unit. (2) Athigh humidity, that is, above 90% the carbon particles in the coating orpaste are moved far apart by the humectant. Thus the inherent filmresistance is of the order of megohms. The difiiculty with this type ofthick carbon paste coating is that if any of the components introduce aparallel resistance which is much less than the basic resistance, thisresistance, especially if the impurities are ionic in nature, decreaseswith increasing humidity. The result is that a maximum resistance isobtained around 90% R. H. rendering the unit worthless above 90%.

In this invention it is a primary object to provide a carbon typehygrometer in which the carbon paste is mixed with a metal powder andthe mixture is applied to a plastic, glass or ceramic strip separatingtwo electrodes and in which the film thickness of this mixture isextremely limited.

A further object of this invention is to provide a carbon typehygrometer in which a carbon paste including a humectant is mixed withpowdered silver and the resultant mixture is sprayed or painted ordipped on a plastic, glass or ceramic strip separating two electrodesand in which the carbon and silver coating is extremely thin.

A still further object of this invention is to provide a carbon typehygrometer with a high positive humidity co-eificient of resistivity andan extremely low electrical resistance.

A still further object of this invention is to provide a carbon typehygrorncter which includes a glass core with an electrode at either endthereof and a thin coating of humidity responsive solution applied tothe glass core between the two electrodes in which said humidityresponsive solution is comprised of a metal powder, a carbon powder, adispersing agent, a binder and a moisture absorbing humectant.

A further object of this invention is to provide a carbon typehygrorneter in which there is a minimum thicl'- ness of film of humidityresponsive solution to reduce the effect of ionic impurities in saidfilm.

A still further object of this invention is to provide a carbon typehygrometer that comprises a glass core with a carbon type humidityresponsive solution coated on said glass core between a pair ofelectrodes and in which :he magnitude of the film thickness is reducedto a minimum to in turn reduce the ionic impurities that mice thestability of the coating in maximum relative humidity.

Other objects of this invention shall be apparent by reference to theaccompanying detailed description.

In using a carbon type hygrometer it is of the utmost importance thatthe carbon fiim that will provide a humidity response must be of aminimum thickness. However, the basic deficiency in using carbon is dueto its low electrical conductivity. Since the electrical conductivity ofmetals is decades higher than that of carbon, it is believed that thecombination of a metal powder with a carbon paste or powder mightenhance the mixture to form a good conductive path between electrodeswhen the material is coated on a non-conductive core such as plastic,glass or ceramic. It has been found that in such a mixture, the metalparticles do not cohere but rather seem to separate and stand off fromeach other. This would indicate an effective break in an electrical pathor chain from one electrode to another. Whereas, the carbon particlestend to cohere and although their electrical conductivity is low, theytend to produce a plurality of conductive paths between electrodes. Themixture of carbon powder and silver thus will produce bridges betweenthe conductive metal particles and in effect the metal particles willassist the conductive path of the carbon particles and raise theelectrical conductivity of each path into which the particles becomechained. Although there is a material chain reaction of the carbonparticles, the metal particles are induced into this chain to shunt outsome of the carbon particles and produce the desired increasedelectrical conductivity between electrodes. Experimenting with variousmetal powders, silver powder has been found to produce a most desirableresult, that is, mixing easily some infinity to bridge and having a highelecwith the carbon powder, having or join with the carbon particlestrical conductivity. Utilizing this new mixture of metal powder withcarbon powder and providing the necessary dispersing agents, binders andhumectant provides a coating mixture that when applied to anon-conductive core to separate a pair of electrodes will produce a lowresistance humidity element with an extremely thin film coating and mostimportant of all this coating produces a tremendously fast response andthe element having a low resistance permits the humidity range of theelement up close to 100%.

Although there is disclosed in this invention a carbon type hygrometerin which a non-conductive core separates a pair of electrodes and inwhich a thin film of humidity responsive solution is applied to saidcore from one electrode to the other and in which this film or core iscomprised of carbon powder, powdered metal, a binder, a humectant and adispersing agent, it is to be understood that the percentages of theingredients may be varied and the particular metal powder to be utilizedmay be varied without departing from the spirit of this invention. It isfurther apparent that other metal powders which do not form superficialoxides in the atmosphere may be similarly used and this invention shallbe limited only by the appended claims.

What is claimed is:

1. A carbon type hygrometer which includes a nonconductive coreseparating a pair of electrodes, a humidity sensitive film coated onsaid core between said electrodes, said humidity sensitive filmincluding a mixture of carbon powder and conductive metal powders in a50-50 mixture, saidmixture mixed in a dilute water solution withhydroxy-ethyl cellulose, poly-oxyethylene, sorbitol and alkyl arylpolyether alcohol.

2. In a carbon type hygrometer according to claim 1 in which the film ofhumidity responsive solution is reduced to a minimum to increase theresponse of the element while keeping the resistance of the elementextremely low.

3. In a carbon type hygrometer according to claim 1 in which themagnitude of the coating thickness is reduced to a minimum withoutafiecting the stability of the coating at maximum relative humidity.

4. In a carbon type hygrometer according to claim 1 in which apercentage of carbon powder to silver powder may be a minimum of 10%silver to a maximum of 90% carbon.

5. In a carbon type hygrometer according to claim 1 in which apercentage of carbon powder to silver powder may be a maximum of 90%silver to a minimum of 10% carbon.

6. In a carbon type hygrometer according to claim 1 in which theconductive metal powder is the precious metals that do-not oxidize inair.

References Cited in the file of this patent UNITED STATES PATENTS2,256,642 Gaut et a1. Sept. 23, 1941

1. A CARBON TYPE HYGROMETER WHICH INCLUDES A NONCONDUCTIVE CORESEPARATING A PAIR OF ELECTRODES, A HUMIDITY SENSITIVE FILM COATED ONSAID CORE BETWEEN SAID ELECTRODES, SAID HUMIDITY SENSITIVE FILMINCLUDING A MIXTURE OF CARBON POWDER AND CONDUCTIVE METAL POWDERS IN A50-50 MIXTURE, SAID MIXTURE MIXED IN A DILUTE WATER SOLUTION WITHHYDROXY-ETHYL CELLULOSE, POLY-OXYETHYLENE, SORBITOL AND ALKYL ARYLPOLYETHER ALCOHOL.